Selective separation of xylenes by crystal size classification

ABSTRACT

A PROCESS IS PROVIDED FOR RECOVERING HIGH PURITY METAXYLENE FROM MIXTURE OF PRIMARILY C8 AROMATIC ISOMER (1) BY COOLING SUCH A MIXTURE TO FORM A SLURRY OF LIQUOR AND DISCRETE META- AND PARA-XYLENE CRYSTALS OF DIFFERENT SIZE AND SHAPE, (2) BY CLASSIFYING CRYSTALS OF THE TWO SPECIES ON THE BASIS OF THEIR DIFFERENT SIZE AND SHAPE AND/OR SETTLING RATES TO OBTAIN A CRYSTAL STREAM ENRICHED IN METAXYLENE, AND (3) BY CONCENTRATING THE META-XYLENE RICH CRYSTAL SOLIDS. PARTIAL MELTING OR COMPLETE MELTING AND RECRYSTALLIZING CAN BE USED FOR FURTHER PURIFICATION TO OBTAIN VERY HIGH PURITY META-XYLENE.

March 19, 1974 A. G. BEMIS ETAL SELECTIVE SEFARATION OF XYLENES BYCRYSTAL SIZE CLASSIFICATION 3 Sheets-Sheet 1 Filed Aug. 11, 1972 m N N 0v $.23 mRw ikd Ill QQN .QRQ raw SEQ wuixu WEN wkk wkwx 8% I so WEN X RN?||Il 3n Q0 2 QQNH swa s/w hum 97 mun/a March 19, 1974 BEMIS ETAL-3,798,282

SELECTIVE SEPARATION OI XYLENES BY CRYSTAL SIZE CLASSIFICATION COMPOS/T/ON F CYCLONE UNDER/W 504/05 VS L/QU/D FLOW SPl/T 85% MX M/FEED Mr wrz-eo 7.5 7; MA v FEED I I I I I I I I I I I I I I I I I I 80 I I I IUNDER FL 0 w 504 m6 Cam os/770M I wz: 7? M7A an are 5 5 73 82.6 87. 590.5 925' 75 I I I I I 76 0F FEED L/Ql/ID 7v u/vaim ww March 19, 1974BEMls ETAL SELECTIVE SEPARATION OF XYLENES BY CRYSTAL SIZECLASSIFICATION Filed Aug. 11. 1972 5 Sheets-Sheet 5 'U.S. Cl. 260-674 A3 798' 282 SELECTIVE SEPARATfON F XYLENES BY CRYSTAL SIZE CLASSIFICATIONAlan G. Bemis, John K. Darin, and Melvern C. Holf,

Naperville, Ill., assignors to Standard Oil Company, Chicago, Ill.

Filed Aug. 11, 1972, Ser. No. 279,853 Int. Cl. C07c 7/14 32 ClaimsABSTRACT OF THE DISCLOSURE A process is provided for recovering highpurity metaxylene from mixtures of primarily C aromatic isomers (1) bycooling such a mixture to form a slurry of liquor and discrete metaandpara-xylene crystals of different size and shape, (2) by classifyingcrystals of the two species on the basis of their different size andshape and/ or settling rates to obtain a crystal stream enriched inmetaxylene, and (3) by concentrating the meta-xylene rich crystalsolids. Partial melting or complete melting and recrystallizing can beused for further purification to obtain very high purity meta-xylene.

BACKGROUND OF THE INVENTION This invention relates generally to aprocess for separating compounds with low boiling point differentials,and more particularly to a process for selectively separatingmeta-xylene and para-xylene by crystal size classification. Morespecifically, however, this invention is concerned with the recovery ofhighly purified meta-xylene, preferably in the range of 95 to 99 percentpurity by crystallizing a mixture of metaand para-xylene containingprimarily C aromatic isomers under controlled conditions to obtain amixture of metaand para-xylene crystals and then separating the metaandpara-xylene crystals by virtue of the differences in their crystal sizeand shape. After concentrating the meta-xylene rich solids, partialmelting or complete melting and recrystallization may be used ifnecessary to attain the desired final meta-xylene composition.

In recent years an increased need has developed for meta-xylene richfeedstocks to be used, for example, in making isophthalic acid andrelated products. On the one hand, little difficulty has occurred ineffecting the separation of ethylbenzene and ortho-xylene from Cmixtures by fractionation, primarily because of the relatively largeboiling point diiferences between those C components and related Ccomponents. 0n the other hand, however, great difliculty has beenexperienced in effectively separating meta-xylene and para-xylenebyfractionation because of the small 0.8 C. boiling point difference whichexists between the components. 7

. As a consequence, a variety of crystallization techniques have beenattempted to separate meta-xylene from para-xylene, but none has provedcommerciallyfsuccessful in selectively producing high concentrations ofmetaxylene. Most commercial para-xylene crystallization processes, forexample, produce meta-xylene rich reject filtrate streams (afterpara-xylene recovery has been accomplished) which contain as much as 8to 13 percent paraxylene. Efforts to crystallize such mixturesto obtainhigh purity meta-xylene, have resulted in the formation of mixtures ofparaand meta-xylene crystals, which at best provide an unsatisfactoryresultant equilibrium composi- 3,798,282 Patented Mar. 19, 1974 beintroduced into the separation system, which, in turn,

requires additional equipment for the subsequent separation and recoveryof this third component. In the case of meta-xylene separation byselective sulfonation, considerable equipment is required merely torecover and reconcentrate sulfuric acid. Likewise, the use of HF-BF formeta-xylene recovery requires the handling of corrosive gases andexpensive special alloys for operating plant equipment.

Clathration techniques, on the other hand, require the mechanicalhandling of approximately 8 to 10 times the mass of extraneous materialsrelative to the component to be separated, and thus, is highly expensiveon the basis of energy requirements alone. Finally, even adsorptiontechniques, in which one isomer component is adsorbed on a solidadsorbent, require large initial inventments in adsorbents that can bereadily inactivated by contaminants contained in the isomer feedstock.

As a consequence of these and various other disadvantages inherent inthe prior art separation processes, a considerable need has developedfor a reliable but less expensive process for recovering high puritymeta-xylene.

SUMMARY OF THE INVENTION It has now been discovered, in accordance withthis invention, that when metaand para xylene containing mixtures ofprimarily C aromatic isomers are crystallized under controlledconditions, a mixture of discrete metaand para-xylene crystals areobtained which diflEer sufiiciently in size and shape to permit theirseparation by classification. The invention may also be applied morebroadly to a mixture of meta-, ortho and para-xylene crystals. It hasalso been found that meta-xylene crystals formed from thecrystallization of a mixture of C aromatic isomers are larger thanortho-xylene crystals, which, in turn, are larger than para-xylenecrystals.

Thus, in accordance with this invention, concentrates of meta-xylene andhigh purity meta-xylene are produced without fractionation by: (1)cooling a mixture of primarily C aromatic isomers containing para-,meta-, and ortho-xylene to a low enough temperature and for a timesufiicient to provide a slurry of liquor and crystals, the crystalscontaining less than 10 percent by weight orthoxylene crystals, (2)physically classifying, in accordance with the relative size and shapeof their respective crystals, meta-xylene, ortho-xylene, para-xylene,and (3) concentrating the meta-xylene rich crystal solids by separatingmost of the filtrate. Partial melting or complete melting andrecrystallization may be used for further enrichment. as for example, to99% meta-xylene.

BRIEF DESCRIPTION OF THE DRAWINGS This invention will be more readilyunderstood by reference to the following drawings, in which:

FIG. 1 is a graphic illustration of the settling rates for paraandmeta-xylene in mixtures of xylenes showing specifically that meta-xylenecrystals, as discussed in more detail in Example 3, settle more rapidlythan para-xylene crystals;

FIG. 2 is a graphic illustration of the feasibility of separatingmetaand para-xylene by means of cyclone separation as described in moredetail in Example 6.

FIG. 3 is a process flow diagram of commercial scale equipment designedto separate metaand para-xylene in accordance with the process of thisinvention.

DESCRIPTION OF EMBODIMENTS benzene and ortho-xylene.

Although the mixture of C isomers will desirably have -metaandpara-xylene present in substantially their eutectic ratio, the inventionmay also be successfully applied :to mixtures other than the eutecticratio. The eutectic ratio of metaand para-xylene varies withconcentration of these two components. For example, in abinary mixturecontaining only paraand meta-xylene, the eutectic ratio for metaandpara-xylene is about 87 to 13 metato para-xylene. In the case of a Cisomer mixture having only a combined metaand para-xylene concentrationof 25 percent, the eutectic ratio is about 91 to 9 metato para-xylene.

In any event, however, it has been determined that highly desirablestarting materials for use in the practice of this invention are thetypical reject filtrate streams obtained from para-xylene recoveryplants. One reason for this preference of reject filtrate streams, ofcourse, is their ready availability from existing para-xylene re coveryplants at a temperature close to that desired in subsequentcrystallization. Furthermore, such reject filtrate streams generallycontain concentrations of metaand para-xylene in approximately theireutectic ratio.

A suitable reject filtrate stream containing a mixture of C isomers thatis useful in the practice of this invention includes about 8 to 13percent by weight para-xylene,

about 8 to percent by weight ethylbenzene and about,

15 to percent by weight ortho-xylene, the remaining portion of themixture being meta-xylene. 'More specifically, a typical startingmaterial for use in this invention would include: about 18 percent byweight ethylbenzene, 10 percent by weight para-xylene, 59 percent byweight meta-xylene and 13 percent by weight ortho-xylene.

Theconcentration of ortho-xylene contained in the mixture is desirablyadjusted to a level below the eutectic ratio of orthoand meta-xylene.This requires the removal of sufficient ortho-xylene so that therequired: amount of meta xylene can be crystallized without firstreaching the ortho-xylene eutectic. It has been determined that areduction in the ortho-xylene concentration below the eutectic ratio oforthoand meta-xylene results in sev eral distinct advantages. First, itreduces operating costs,

since crystallization of a C mixture containing substantial amounts ofortho-xylene is more costly. Secondly, and more importantly, a reductionin the quantity of orthoxylene crystals formed upon crystallization andpresent in the mixture of discrete metaand para-xylene crystals,-

or, preferably, the complete elimination of ortho-xylene crystals, ofcourse, simplifies the classification step, since only two types ofdiscrete crystals (metaand para-xylene crystals) need be classified. Anymeans to remove orthoxylene can be employed, but fractionation is thesimplest and preferred method. If fractionation is employed, ortho-.xylene removal is most advantageously accomplished before, rather thanafter, para-xylene crystallization.

After selection and preparation of a suitable mixture of primarily Cisomers, the mixture is cooled in a crystallizer for a time and at atemperature sufiicient to yield a slurry of liquor and discrete crystalsof the desired solids concentration; the mixture of discrete crystals soformed containing less than 10 percent by weight ortho-xylene.Preferably, however, the mixture of C isomers is cooled for a time andat a temperature above which no C isomers other than meta-xylene andpara-xylene crystallize. In this way, as discussed previously, only twodiscrete types of crystals, e.g., metaand para-xylene, will be presentin the solid crystal phase and, thus, ultimate separation byclassification is simplified.

Typical cooling temperatures used in the practice of this invention varyfrom about 65 to C. Naturally, the percent solids formed duringcrystallization varies as the feed composition, cooling temperatures,and crystallizer residence time vary. Preferably, however,crystallization is carried out until a manageable slurry of liquor andsolid crystals results, e.g., a slurry having a solids content inthe'range of 10 to 40 percent and, preferably, 10 .to 30 percent. Slurryof lower solids concentration can be handled by the process of theinvention. However, operation of the process is more costly at lowersolids concentrations.

Once discrete crystals, differing in size and shape, are formed, anumber of size and shape classification techniques can be employed toeffect the ultimate recovery of meta-xylene. Classification can beaccomplished for example, by screen, cyclone, centrifuge, or settlingtechniques, or for that matter, by any known physical or chemicalclassification technique which can separate discrete crystals bydifferences in their size, shape or settling rate.

It has been discovered, therefore, that the process of this inventioncan be effectively used to separate metaand para-xylene in a commercialscale operation, without the need for third components being introducedand then removed from the separation system. The process is high.- lycompatible with existing para-xylene recovery facilities and can easilytake advantage of existing refrigeration capacity in such facilities.Moreover, no corrosive components, nor expensive equipment, are requiredto commercialize the process, and yet high purity meta-xylene, in therange of to 99 percent by weight, can be recovered.

The numerous advantages of this invention will be further described andillustrated with the aid of the following examples. All parts andpercentages are by weight unless otherwise indicated.

Example 1 Table I summarizes data from actual tests in which typicalmixed xylene feeds were crystallized to obtain mixtures of metaandpara-xylene crystals, and in which the crystal sizes were measured. Thelaboratory crystallization vessel was a 1.5-liter glass vessel jacketedso that coolant could be circulated around the outer walls. Stirring wasaccomplished by nylon scrapers whichrotated with spring tension-againstthe inside walls to prevent the buildup of any crystals. The pilot plantcrystallization vessel was a 12-gallon, stainless steel vessel fittedwith jackets through which coolant was circulated and fittedin- ,trnallywith scrapers to provide stirring and to prevent buildup of crystals onthe walls. Both the laboratory and pilot plant vessels were designed tosimulate'typical commercial para-xylene crystallizers.

.Crystal sizes were measured by withdrawing samples from the vessels andcharging them to a specially designed photomicroscopy unit (Photo-Dewar)in which the crystals could be maintained at the temperature at whichthey were grown and photographs of samples taken. The photographs werethen enlarged and in this set of experiments crystals sizes were made bymeasurement of the crystals in the photographs. Table I summarizes theconditions of the crystallization and compares the average crystal sizeofthe para-xylene and the meta-xylene crystals which co-crystallized. Itshould be noted from these data that the average size for meta-xylenecrystals is greater by a factor of 30 to 300 than the averagepara-xylene crystal size. Thus, any screening device with suitableapertures can effect a classification of this crystal mixture to form aconcentrate richer in meta-xylene than the original crystals present inthe mixture and another concentrate correspondingly richer inpara-xylene.

6 smallest crystals, and by repetitive partial melting and slowrecooling which produced the largest crystals.

TABLE I.CRYSTAL SIZES FROM MIXED XYLENE CRYSTALLIZATION Crystal- Averagecrystal size (micronfi Type oi lization A Size Feed operation temp., C.F. Para-xylene Meta-xylene ratio 4 A... Batch? 69. 5 2 100 x 300 x 10030 A- Continuous-.. 70. 0 16 100 x 1,000 x 250 167 A- 0 70. 0 21 100 x10 700 x 250 175 A.-. Batch 70.0 14 150 x 10- 800 x 300 160 A.-.Continuous.... 70. 6 26 100 x 10 900 x 300 270 A--- Batch 68.7 2 100 x10 600 x 150 75 Continuous-.. 67. 3 7 100 x 10 800 x 150 120 Batch 75.52 100 x 10 300 x 150 45 75.5 10 100x15 700x200 93' 75. 5 16 150 x 15 1200 x 400 17 76. 0 24 100 x 15 1,300 x 350 303 70. 0 16 150 x 15 1,500 x400 267 70. 0 29 100 x 15 1,200 x 400 320 70. 0 23 150 x 15 1,200 x 400213 1 Feed A=18.4% 50.9% MX, 16.3% 0

ga n, 9.0% PX, 60.0% MX, 12.5% ox. Feed B=25.2% EB, 7.2% PX,

I Ttelmperature diflerence between coolant temperature and crystallizersolution temra e. pet Thickness of crystals is estimated to be about thesame as width (or PX crystals, about 10-50 microns tor MX crystals.

Size ratio=(width x length of MX crystals) /(width 1 length oi PXcrystals):

Example 2 Tests were run with mixed xylene feeds in a pilot plant toobtain actual crystal size distribution with respect to theco-crystallized metaand para-xylene isomers when crystallization wascarried out continuously and under simulation of typical commercialoperation. These data are summarized in Table II and unexpectedly showthat nearly all of the para-xylene crystals are less than 200 microns inlength whereas about 95% of the meta-xylene crystals by weight aregreater than 200 microns in length. These crystal size distributionswere obtained by withdrawing samples from the crystallizer, chargingthem to the Photo-Dewar and taking photographs of the mixed crystals.Crystal size distributions were then obtained by using a model TGZ3Zeiss Particle Size Counter. These data are those used to showthe-typical crystal size ranges in FIG. 1 (Example 3) and also show thatthe use of any screening device with suitable apertures can efiect aseparation of metaand para-xylene crystals from such sl urriesof-mixtures.

The settling rates of meta-xylene crystals and paraxylene crystals 'weremeasured in the laboratory in an 18- inch tube with a one-inch diameterwhich was immersed in a constant temperature bath at identicaltemperature with the crystallizer. The crystals were observed through asmall telescope mounted on a scale so that it could be raised orlowered. The settling of crystals was measured by following one crystal,and recording the time it took to fall a standard distance, usually 10cm. In a given run, the settling rates of 100 to 200 crystals weremeasured.

The procedure followed was to transfer some of the crystals from thecrystallizer to the sample vessel in a thermostated bath. Some of thesecrystals were photographed 'and their sizes measured, and some weretransferred into the settling tube (within the same thermostated bath).Ten diiferent runs were made-six for meta-xylene, and four-forpara-xylene. In each case, at least 100 settling measurements were madeand 20 photographs taken.

' To estimate the settling rate for each size particle, the settlingrates for each run were listed in order of rate and divided into tenpercentile groups. Similarly for each TABLE Ill-CRYSTAL SIZEDISTRIBUTIONS OF META- AND PARA-XYLENE IN PILOT PLANT SLUR- RIES SAMPLEDFROM THE BOTTOM OF THE CRYSTALLIZER Residence Feed composition ResidenceCrystal size distribution-wt. percent 0! the total solids between time,time, 7 the listed intervals (microns) iresh teed, Percent Percent AT 1external remin. PX MX 1 cycle, min. 0-100 100-200 200-300 300400 400-500500-600 600-700 Meta-xylene v Para-xylene M V 45 e 5e 24 2.0 us "25.2 459 58 38 2.0 73. 1

I All runs are continuous.

1 Difierence between coolant temperature and slurry temperature.Crystallization temperatures were approximately 99 F.

I Residence time in crystellizer due to external recycle.

Example 3 Table III summarizes data on the settling rates of paraxylenecrystals and meta-xylene crystals as a function of crystal size. Inthese tests, feeds similar to those used in Example 1 were employed,but, in one case, meta-xylene was replaced with ethylbenzene so thatonly para-xylene would crystallize and in the other case, para-xylenewas replaced with ethylbenzene so that onlymeta-xylene wouldcrystallize. Thus, crystals of only one component formed.

and suspersaturation conditions of the system.

In a given test, crystals of one size were made in the laboratorycrystallizer .as described in Example '11. It was l of the meta-xylenecrystals are larger than 200 microns I and, thus, settle more rapidlythan para-xylene crystals of The temperature employed was about-73? C.Difierentj which about are smaller than 200 microns. Thus, sizes ofcrystals were'grown'by controlling the'nucleatron most of themeta-xylene crystals will settle at a much faster ratethan theaccompanying para-xylene crystals and separation by any sedimentationdevice such as a centrifuge or an elutriation tower will indeed providesepapos's'ible to'makevalriousiizes of crystalsby'either'fi id ration orconcentration of the xylene isomers with respect crystallization fromshock nucleation, which produced the 7 to each other.

Values of settling rates (cm./sec.) and size (len decile of the crystalshadgth in mm.) which each Decile -10 10-20 -30 -40 -50 -60 -70-70-80-90. -100 Para-xylene crystals V .08 .033 .037 .037 .038 .04 .06 .062.05 .05 .05 .05 .1 .1 .1 .1 .1 .035 .05 .07 .078 .1 .13 .2 .3 .5 .05.075 .1 .1 .15 .2 .2 .25 .37 .03 .04 .05 .05 .06. .075 .08 .09 .15 .05.075 .1 .1 .13 .17 .2 .2 .32 .02 .02 .02 .02 .03 .03 .03 .04. .05, .03.05 .05 .05 .1 1 .1' .17 .25

Meta-xylene crystals l Deciles from settling rates and deciles fromcrystal sizes are set in 1 to 1 correspondence.

Example 4 1 to 3 rpm. so that partial settling occurredaAt that pointthe upper 78% of the vessel contents comprising the clear liquor andslurry was withdrawn overhead. The remaining lower portion was thenallowed to settle further and the liquid withdrawn from this materialthrough a small aperture, the crystals themselves acting as a partialfilter. Some smaller crystals were also withdrawn in this process. Thereresulted 30 grams of a crude solids-wet with mother liquor. This crudesolids was found tocontain 10.7 grams of a meta-xylene/para-xylenecrystals c'ontaining 95.4% meta-xylene" and 4.6% para-xylene. Theseresults demonstrate that the original feed mixture of 12% para-xylene/88% meta-xylene, approximately a eutectic composition, could be enrichedto a mixture-containing considerable meta-xylene in excess of thepara-xylene.- From such a mixture asobtained as the final product, morethan half of the meta-xylene could be recovered as. pure meta-xylene bysimple recrystallization. q W

TABLE IV '-v foundf-to 'be 31,9 grams comprising;22% para-xylene .and

summarized in Table V. A portion of this slurry was fed to a 5"International Centrifuge-Chemical Model, with a perforated basket linedwith filter paper. The centrifuge .had been modified so that the bowlwas in a constant temperature box which could be maintained at the sametemperature as they crystallizer. .There was obtained 381 grams ofmother liquor, 30 grams of a solidproduct containing smaller amounts ofmother liquor. The solids content of this product was found, to be 27grams which contained 13.2% para-xylene and 86.8% meta-xylene, andactually shows a slight enrichment in favor of paraxylene for thisportion of the experiment. In the second part of the example, the samecentrifuge was used except that the perforated bowl was replaced withasolid bowl which was filled with ethylbenzene and rotated at arelatively low speed-of about 800- to 1000 revolutions per minute: Theremaining slurry was then slowly charged to this bowl. the largercrystals settling to the outer edge of the-:bowl and the excess liquidflowing over the lipof the bowl alongwi'th smaller. crystals. Thereresulted an-overflow of 463 grams and an underflow remaining inthezbowlof 141 grams. The solid contents of the overflow was 78% meta-xylene,.highly,enriched in para-xylene compared to the solids in the original'slur ry. Thesolid'contents of the underflow was found to be. 35.6 grams-and Separation. h me i e w e comprised 8.4% para-xylen't'a and 91.6%meta-xylene. u Thus, .the original approximately eutectic mixture of 12%Decantationof Solid contents with V V I 'Iinderflow 1' motherliquorremoved' Settling "d T l ru 0 otal in After Feed OverflowUnderflow" Drawofl' solids 'underfiow" decanting Total welght,g 1,100773 11 ,-q -8 30 v18v 10.7 Ethylbenzene, percent. 14. 5 16. 1 14.7 15.612 .3 Parsxylene, percent. 8.8 8.4 8.0 8.4 6.8 7.3' l 4.6 Metaxylene,percent- 64. 5 63. 3 66. 0 64. 1 71. 6 92. 7 9. 54 Orthoxylene, percent11. 5 12. 3 11- 2 11- 9 9. 3 3

Example 5 para-xylene, 88% meta-xylene in the feed mixture had In thisexample, 1100 grams of a typical feed wascrystallized-at 65.9 C. inthelaboratory crystallizer dejb e'en enriched'in rneta-xylene to 91.6%.From such a mixture about /a of the meta-xylene can be recovered inscribed in Example 1. The results of the experiment are 65 high puritybysimple recrystallization. TABLE V.ISEPARATIQN or META xYLENnBY CRYSTALSIZE I Solid bowl centrifuge Centrifugal filter p Overflow Underflow l gSolids exsolids ex. solids ex. Mother Total mother I v 1 g mother motherFeed liquor solids liquor Feed Overflow liquor Underfiow liquor Totalweight, g 381 30 27 463 31. 9 141 35. 6 Ethylbenzene, percent 14.5 15.91.9 14.5 15.0 12.3 Paraxylene, percent- 8.8 8.6 12.7 13. 2 8.8- 9.6 g 228.8 8. 4 Metaxylene, percent- 64.5 62. 6 84.2 86.8 64. 5 64.1 v 77 69. 591. 6 11.7- 12.0 1.2 7 11.3 9.3

Orthoxylene, percent ethylbenzene, 8.8% para-xylene, 64.5% meta -xyleneand 11.5% orthoxylene and crystallized at -94 F., one would obtain a.slurry containing 24.7% solids comprising about 87% meta-xylene and 13%para-xylene. If this slurry were then fed to a 40" x 60" BirdScreen-Bowl centrifuge, 64.6% of the para-xylene would be recovered assolids, the remainder being rejected in the overflow; and 92.9% of themeta-xylene would be recovered as solids, tlie remainder also beingrejected in the overflow. The composition of these solids would be 90.6%metaxylene and 9.4% para-xylene and suitable for recrystallization orpartial melting to obtain purer meta-xylene. These data then illustratethat commercial and available centrifuge equipment can be used to elfectthe separation of meta-xylene from para-xylene by classification.

1 It should be understood that much greater enrichment of meta-xylenecan be obtained through the use of commercial centrifuges by merelyvarying therotational speed, e.g., centrifugal force, of the centrifugeand/ or the slurry feed rate for the xylene mixture. For example, it iscontemplated that a centrifuge operating under the same classificationconditions as the cyclone described in Example 6would result in theenrichment of a feed composition having 87.5 percent meta-xylene to acomposition having 97.5 percent meta-xylene.

: WEIGHT SAMPLE ANALYSES (MOLE PERCENT) Feed Cake

Toluene Ethylbenzene. Para-xylene Meta-xylene Ortho-xylene Example 9These tests, summarized in Table VIII, were carried out to show thatsimple screening can also be used to obtain concentrates of meta-xylene.In Test 1, the feed was crystallized at 70 C., in the laboratoryapparatus described in Example 1, to produce a slurry whose solids hadthe composition 16.6% para-xylene-83.4% meta- PERCENT Microns 1-100100-200 200-300 300-400 400-500 500-600 Total bowl centrifuge) (1; PX 5980 92 96 100 100 100 (2 MX 59 80 92 96 100 100 Typical weight percentcrystal size distribution, Run 1 (s01ids=87/13 MX/PX) (3) PX 73.1 26.9 I100 (4) MK---" 0. 1 2. 6 12 28 31. 3 19. 5 6. 5 100 Weight percentsolids recovered (5; PX-..-- 43.1 21.5 64.6 (6 MX 1. 5 9. 6 25. 8 30.019. 5 6. 5 92. 9

See the following: I Solids recovered: 2

meta-Xylene (87.0) (92.9), per 80.8 para-Xylene (13.0) (64.6), percent8.4 'Ijotal, percent 89. 2 Composition:

meta-Xylene, percent 90. 6 para-Xylene, percent- 9. 4

Example 8 This example demonstrates that high purity metaxylene' can beobtained by recrystallizationof the metaxylene solids obtained by thedisclosed crystal classification. In this way, one can obtain greatlyenriched high purity nieta-xylene by recrystallization of the metaxyleneobtained by crystal classification.

Approximately 1100 grams of an 86.2 mole percent meta-xylene solutionsuch as could be; recovered from para-xylene reject filtrate bycrystallization-classification was cooled in a glass vessel to 64.-9 F.After 96 xylene. The slurry was transferred to a vibrating screen, whichwas enclosed in a refrigerated compartment held at C. The screen hadpores of 250 microns and a vibrating angle of 50 was used. The slurrypassing through the screen was collected, and the crude solids remainingon the screen as well as the crude solids which overflowed the screenwere collected. The overflow actually contained about 48% solids, whosecomposition was 95% meta-xylene, 5% para-xylene, thus demonstrating adramatic enrichment by screen separation. v

TABLE VIII Test 1 Solids composition Solids Solids slurry compositioncomposition Crystallizer from of feed crystallizer Overflow overflowUnderfiow' underflow Percent: I i I I 12. 1 Solids in stream 11 Test 217. 0 13. 3 18. 9 9. 4 18 7. 7 10 10. 3 31 52.0 66 61.2 47.3 42 OX 21. 516 17. 5 10 23. 5 -27 Weight (grams) 1, 280 430 842 Percent solids instream 13 In Test 2, the same apparatus was used to produce a slurry at-73 C. The solids had a composition of 18% para-xylene, 66% meta-xyleneand 16% ortho-xylene. This slurry was fed to the screen with 250 micronpores, which was thermostated at -73. A vibrating screen a proper slurryconcentration.

via line 24 back to the first stage of the crystallization. 'Rejectfiltrate The remainder of this reject is fed angle of 40 was used forthis test. The overflow solids 7 from the first stage centrifuge 1s fedby line 25 to crystalhad a compositlon of para-xylene, 80% meta-mlizer.26 which is operated at amini num t emperature of xylene, and 10%ortho-xylene aga demonstl'atlng a about -77 C. and in which is grown aslurry consisting significant enrichment in meta-xylene content byscreenof a mixture of meta-xylene and para-xylene crystals. hilt r1180demonstfatmg h meta-Xylene can he 10 The slurry from thiscrystallization system is. fed by line separated from both para-xyleneand ortho-xylene by 27 to a screenbowl-centrifuge in which the smallercrystals this technique. and the liquid are removed via line 39 and asolid enriched The Scope and a e f the lnventlon can best be inmeta-xylene is fed by line 29 to melt tank 30. The melted lllustl'ahid ythe yp example of l'Q use 5- mixture of meta-xylene and para-xylene richin meta-xytrated In E and COmPOSItIOIIS for the lenerelative to theeutectic mixture is then fed by line 31 stfeflms are summarlled 111Table fr95h feed Xylene, to crystallizer 42 which is operated at atemperature above -YP Q Y from an extracted reformate, 1S fed to isysiem the eutectic of the contained meta-xylene and para-xylene. km 1and combmeq Wlth the emuent of an Pure meta xylene is thus crystallizedand the slurry is fed igh j Streams are fed dlstlliauon by line 33 to acentrifuge 34. The solid product which is ere ow mg g h i zfi s sue 2:removed by line 35 is substantially pure meta-xylene. The Wa enzene paranap e as are a en I reject filtrate leaves the centrifuge and a portionof it may head and are removed via line 3. A bottoms stream then h f eed via line 36 back to the crystallizer to mamtam a passes v1al1ne 4into column 5 where the contained ethyldesir ble slurr concentration Thremainder fed b benzene, substantially all of the meta-xylene andparalin back Stan. 26 t filtrat xylene and part of the ortho-xylene aretaken overhead Ce las 28 1 32 whe P g d and fed to the crystallizationsystem by line 10. A bottoms 6 a S t i re 3 15 d stream comprisingortho-xylene and heavier aromatics is e or f ccfncen Ia 6 i me an 6 fedby line 6 into distillation tower 7. C and heavier hne to an {somenzer40 i j F xylenes are components are removed through line 8 and an orthoxy converted substantially to an equilibrium m1xture of xyllene richmaterial is fed via line 9 to the isomerization sysenes fed f f f Intothe tem. The xylene concentrate from distillation tower 5 and ThemYentlon not f to the Partlcular from which a part of the ortho-xylenehas been removed, is men! re desc 'lbed, and 1t wlll be ev1dent to onekilled combined with a recycle from line 24 and fed to crystal- In theafilhat Various f= y be made which lizer 1 1 which may be one or morecrystallizers in series. fa l withln the scope of this invention. Forexample, the The final temperature of this crystallization is about 66Screen-bowl cehtl'lfuge 28 could replaced y one C. The slurry resultingfrom the crystallization is fed from more cyclones or hydroclones insenes followed by a cenline 12 to centrifuge 13 from which a cake richin paratrifuge. Alternatively, instead of melting the entire solidsXylene is fed by line 14 to melt drum 15. The melted product in meltdrum 30, partial melting could be emmixture is then fed by line 16 to asecond stage crystallizer ployed so that only the contained para-xyleneand some 17 operated at about 0 C. The slurry obtained from thismeta-xylene is melted leaving a resultant slurry of purer crystallizeris subjected to centrifugal separation in cenmeta-xylene to be feddirectly to centrifuge 34 without the trifuge 19 to provide a solidsmaterial comprising subintervening crystallization step of crystallizer32. It is also stantially high purity para-xylene which is removed byevident that the ortho-xylene concentrate obtained in line line 20. Aportion of this material can be melted and 9 could be processed furtherto obtain pure ortho-xylene recycled via line 21 as wash. The rejectfiltrate from as a commercial and saleable product.

TABLE IX Stream number 1 3 j 6 8 Stream description Fresh teed Lighthydrocarbons Feed to tower 7 Heavy hydrocarbons Wt; Lbs/hr. Wt. percentLbs/hr. Wt. percent LbsJhr. Wt. Percent Lbs/hr. percent 410 1.00 4,82971.28 451 0. 32 0 0.0 0 0.00 1,272 19. 77 106 0.07 0 0.0 Ethylbenzene.-6,140 15.00 450 6.04 23,042 10.17 0 0.0 Meta-xylene--. 18,317 44.70 1121.65 62,496 43.85 0 v. 0.0 Ortho-xy1ene.--. 7,417 19.10 0 v 0.0 30,80821.62 13 1.70 Para-xylene 9, 154 19. 90 112 1. 23, 626 16. 5s 0 0Cghydrocarbonsun 488 1.19 0 0.0 1,311 0.92 1,156 63.45 C10hydrocarbons.-- 45 0. 11 0 0. 0 670 0. 47 635 34. 35

Total 40, 977 100. 00 6, 775 100. 00 142, 500 100. 00 1, 322 100. 00

Steam number 9 10 20 25 Steam description- OX concentrate Feed to PXcryst. train PX product Feed to MX-PX cryst.

. 2 I v, I I wt. Lbs./hr. Wt. percent Lbs/hr. Wt; percent I LbsJhr. Wt.percent LbsJhr. percent Components, lbs./hr.: g

C1 hydrocarbons 0 0.0 451 I 0.35 0.4 0.0 450 0.40 08 naphthenes. 0 0. 0106 08 0. 4 0. 0 105 0. 09 Ethylbenzene 2 0. 02 23, 040 13. 04 23 0. 1623, 01s 20. 23 Meta-xylene. 703 i 5.78 61,743 4s. 34 so 0. 57 61,66754.21 Ortho-xylene 11,359 Y 93. 34 18,785 14 71 7 0.26 18,749 16. 4sPara-xylene 1 0.87 23, .514 18.-41 13,835 98.99 9,675 8.51 Cuhydrocarb011s 0 0.0 91 0.07 0.4 0.0 0.08 010 hydrocarbons V i' 0 t 0.0 00.00 0.0 0.0 0 0.00

Total 12,170 100.00 127, 730' 100.00 13, 976.2 100.00 113, 754

'complished by one or more stages of screening. 7.

' TABLE IXContinued Stream numbernn... v a 31 s9 1 40 42 I r I Rei. fil.from Stream descrlption........... MX product MX-PX recycle 1 MX-PX unitIsom. feed Isom. efiiuent Wt. Wt. Wt. Wt. Wt. LbsJhr. percent LbsJhr.percent Lbs./hr. percent Lbs./hr. percent Lbs/hr. percent Com onents,lbs. r.:

(i, hydrocarbo iis 0 0. 0 9 0. 06 450 0. 47 388 0. 36 3, 116 2. 90 C9naphthenes. 0 0. 0 2 0. 01 105 0. 11 103 0. 794 0. 74 Ethylbenzene.-- 0.03 435 2. 96 23, 013 24. 22 20, 372 19. 04 16, 435 15. 29 Meta-xylene-18, 604 99. 22 12, 692 86. 33 43, 063 45. 33 48, 914 45. 72 5, 312 42.14 Orth0-xylene. 23 0. 12 334 2. 27 18, 726 19. 71 26, 996 25. 23 22,816 21. 22 Para-xylene 118 O. 63 1, 229 8. 36 9, 557 10. 06 9, 573 8. 9517, 312 16. 10 Cu hydrocarbons 0 0. 0 1 0. 01 90 0. 09 423 0. 40 1,079 1. 00 C16 hydrocarbons 0 0. 0 0 t). 0 0 0. 00 216 0. 20 656 0 61Total 18, 750 100- 00 14, 702 100. 00 95, 004 100. 00 106, 982 100. 00107, 520 100. 00

We claim:-- 14. The process of claim 12 wherein high purity meta- 1. Aprocess for separating meta-xylene and meta-xylene concentrates from aliquid mixture of primarily C aromatic isomers which comprises: I

I selecting a mixture of C v isomers containing metaxylene andpara-xylene in substantially their eutectic ratio;

cooling said mixture of isomers for a time and at a temperaturesufficient to form a slurry of mother liquor and crystals, said crystalscomprising discrete meta-, paraand ortho-xylene crystals and containingless than 10 percent by weight ortho-xylene crystals, and saidcrystallization taking place in the presence of an undiluted motherliquor which is less dense than said crystals; and

selectively concentrating said discrete meta-xylene crystals from theremaining crystals by crystalclassification, in the presence of theirnaturally occurring, undiluted mother liquor.

2. The process of claim 1 wherein the mixture of C isomers containingmetaand para-xylene insubstantially their eutectic ratio is about 8 to13 percent by weight para-xylene, about 8 to 20 percent byweightethylbenzene, about 15 to 25 percent by weight ortho-xylene, and

the balance being meta-xylene. v

3. The process of claim 1 wherein the cooling of the mixture of isomersis carried out at a temperature'in the range of about 65 to 9-0 C.

4. The process of claim 3 wherein the cooling of'said mixture of isomersis carried out for a time sufficient to solids.

5. The process of claim 1 wherein said discrete meta-, paraandortho-xylene crystals are separatedby physical complished by one or morestages of gravity settling.

7. The process of claim 5 wherein classification is accomplished by oneor more stages of elutria'tion.

8. Theprocess of claim 5 wherein classification is jac- 9. The processofclaim 5 wherein crystal classification is carried out by subjectingsaid mixture of discretecrystals to the action of centrifugal forceinoneor more stages.

10. The process of claim 9 wherein the centrifugal force classificationaccording to individual crystal sizes and xylene is'recovered from theenriched solids by partially melting to obtain para-xylene and some ofthe meta-xylene in a liquid state and separating them from solids.

15. The process of claim 1 wherein substantially all the ethylbenzenecontained in said mixtures of C aromatic isomers is removed prior tocrystallization of the para-xylene-meta-xylene mixture.

16. The process of claim 1 wherein substantially all of the ethylbenzeneand substantially all of the ortho-xylene contained in said mixture ofisomers is removed from the feed xylenes prior to crystallization ofpara-xylene and meta-xylcne-para-xylene mixtures.

17. A process for separating meta-xylene from a liquid mixture of Caromatic isomers which comprises:

selecting a mixture of C isomers containing metaand para-xylene insubstantially their eutectic ratio and ortho-xylene at a. concentrationbelow the eutectic ratio of metaand ortho-xylene;

cooling said mixture of isomers for a time and at a temperaturesufficient to crystallize metaand paraxylene alone without crystallizingortho-xylene, said metaand para-xylene crystals having a density greaterthan that of their naturally occurring mother liquorjand separating bycrystal classification said meta-xylene crystals from said para-xylenecrystals in the presence of their naturally occurring undiluted motherliquor 18. The process of claim 17 wherein said mixture of C isomers isprepared by adjusting the concentration of ortho-xylene contained in amixture of C isomers, having metaand para-xylene in substantially theireutectic ratio, to a concentration below the eutectic ratio of orthoandmeta-xylene.

19. The process of claim 18 wherein the concentration ofiortho-xyleneisadjusted by fractionation.

20. The process of claim 17 wherein the mixture of C isomers containingmetaand para-xylene in substantially their eutectic ratio is about 8 to13 percent by weight para-xylene, about 8 to 20 percent by weightethylbenzene, about 15 to 25 percent by weight ortho-xylene, and thebalance being meta-xylene.

' 21. The process of'claim 17 wherein the cooling of the mixture ofisomers is carriedout at a temperature in the is applied in a solid bowlor a solid-screenbowl centrifuge.

11. The process of claim 9 wherein the centrifugal force is applied byone or more cyclones arranged in ries by filtration or bycentrifugation.

13. The process of claim 12 wherein the recovered solids are furtherenriched in meta-xylene 'by subjecting the crystals to total melting andrecrystallization.

range of about 65 to C.

22. The process of claim 21 wherein the cooling of said mixture ofisomers is carried out for a time sutlicient to provide a slurry ofliquor and crystals having a solids content in the range of about 10 to40 percent by weight solids.

23. The process of claim 17 wherein said classification is carried outaccording to the relative crystal sizes, shapes and settling rates ofmetaand para-xylene crystals.

24. The process of claim 23 wherein classification is accomplished byone or more stages of gravity settling.

25. The process of claim 23 wherein classification is accomplished byone or more stages of elutriation.

26. The process of claim 23 wherein classification is accomplished byone or more stages of screening.

27. The process of claim 23 wherein crystal classification is carriedout by subjecting the mixture of crystals to the action of centrifugalforce.

28. The process of claim 27 wherein the centrifugal force is applied ina solid bowl or a solid-screenbowl type centrifuge.

29. The process of claim 27 wherein the centrifugal force .is applied byone or more cyclones in series.

30. The process of claim 27 wherein the solids enriched in meta-xyleneare recovered from their respective slurries by filtration orcentrifugation.

31. The process of claim 17 wherein substantially all the ethylbenzenecontained in said mixture of isomers is removed prior to crystallizationof the meta-/paraxylene mixture.

32. The process of claim 17 in which substantially all of theethylbenzene and substantially all of the orthoxylene is removed fromthe feed xylenes prior to crystallization of para-xylene and themeta-lpara-xylene mixtures.

References Cited UNITED STATES PATENTS 3,544,646 12/1970 Broughton etal. 260-674 3,029,278 4/1962 Spiller et al. 260-727 3,277,200 10/1966Smith et al. 260-674 2,622,115 12/1952 Carney 260-674 2,777,888 1/1957Hoff et al. 260-674 2,884,470 4/1959 Harrison et al. 260-674 DELBERT E.GANTZ, Primary Examiner C. E. SPRESSER, JR., Assistant Examiner US. Cl.X.R.

UNETED STATES PATENT OFFICE, CERTIFICATE OF CORRECTION Patent No.3,798,282 Dated March 19, 1974 llnvent fl Alan G. Bemis, John K. Darin GMelvern C. Hoff It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Col. 2, line 24, "inventments" should be "investments, per specificationpage 2 line 29.

Col. 2, line 62, "this" should be "the" per specification page 4, line2.

Col. 5, line 23, "to should be "of" per specification line 32, page-9.

Col. 9, line 74, "This" should be "Thus" per specification page 22, line25.

C01. 14, line 16, "42" should be "32" per specification page 29, line15.

Table 1, Col. 5, "Size ratio" line 10 -"17" should be "107" per Table 1"Size ratio" page 9 line 10 of Table.

Table VIII .Solids in Stream "11" should be "21", 5th line in table, perspecification, page 27, line 5 in table.

Table IX line 5, "13" should be "31" per specification page 31, line 5in Table.

Signed and sealed this 12th day of November 1974.

(SEAL) Attest McCOY GIBSON JR. C. MARSHALL DANN Attesting OfficerCommissioner of Patents FORM po'wso (w'sg) uscoMM-oc scan-poo U,S.GOVERNMENT PRINTING OFFICE l9, 0-356-33.

